feat: add SD card font support with on-device download and web management

Add a complete SD card font subsystem that enables users to install and
use custom fonts beyond the three built-in families. This combines the
back-end firmware support (#1327) with the font configuration, build
pipeline, CI distribution, and user-facing management UI (#1392).

Core font system:
- Custom .cpfont binary format (v4) with multi-style support (regular,
  bold, italic, bold-italic) packed into a single file per size
- On-demand glyph loading from SD card with two-pass prewarm rendering
  to bulk-read glyphs per page, achieving near-flash performance for
  Latin text (~697ms vs ~681ms) and viable CJK rendering (~32% slower)
- Persistent advance cache for layout measurement without SD I/O
- Overflow ring buffer for glyph cache misses during rendering
- Memory-conscious design: only advance tables kept in RAM; glyph
  bitmaps, kern tables, and ligatures loaded on demand from SD

Font management:
- On-device WiFi download from GitHub Releases with manifest-based
  discovery, install/update detection, and progress UI
- Web interface font upload, listing, and deletion via /fonts page
- Manual SD card copy to /fonts/ or /.fonts/ directories
- Font selection integrated into Settings > Reader > Font Family

Build pipeline:
- Declarative YAML config (sd-fonts.yaml) as single source of truth
  for the 17-family font library (serif, sans, mono, accessibility)
- Python converter (fontconvert_sdcard.py) for TTF/OTF to .cpfont with
  FreeType rasterization, class-based kerning, and ligature extraction
- Parallel build orchestrator with variable font instance extraction
- CI workflow publishing versioned + stable releases to a dedicated
  crosspoint-fonts repository with auto-incrementing revision tags
- Centralized version constants (cpfont_version.py) shared across
  build tooling and CI, with firmware headers as manual sync points

Additional fixes:
- CJK characters no longer get hyphens inserted at line breaks
- Advance table eliminates 30+ second stalls during CJK section
  indexing for paragraphs with >512 unique codepoints

Closes #930

Co-authored-by: Zach Nelson <zach@zdnelson.com>
Co-authored-by: Justin <itsthisjustin@users.noreply.github.com>
Co-authored-by: jpirnay <jens@pirnay.com>
Co-authored-by: mcrosson <kemonine@kemonine.info>

src/main.cpp

@@ -22,6 +22,7 @@
 #include "MappedInputManager.h"
 #include "OpdsServerStore.h"
 #include "RecentBooksStore.h"
+#include "SdCardFontSystem.h"
 #include "activities/Activity.h"
 #include "activities/ActivityManager.h"
 #include "activities/settings/SdFirmwareUpdateActivity.h"
@@ -34,7 +35,8 @@ MappedInputManager mappedInputManager(gpio);
 GfxRenderer renderer(display);
 ActivityManager activityManager(renderer, mappedInputManager);
 FontDecompressor fontDecompressor;
-FontCacheManager fontCacheManager(renderer.getFontMap());
+SdCardFontSystem sdFontSystem;
+FontCacheManager fontCacheManager(renderer.getFontMap(), renderer.getSdCardFonts());
 
 // Fonts
 EpdFont notoserif14RegularFont(&notoserif_14_regular);
@@ -195,6 +197,8 @@ void enterDeepSleep() {
   powerManager.startDeepSleep(gpio);
 }
 
+void ensureSdFontLoaded() { sdFontSystem.ensureLoaded(renderer); }
+
 void setupDisplayAndFonts() {
   display.begin();
   renderer.begin();
@@ -225,6 +229,10 @@ void setupDisplayAndFonts() {
   renderer.insertFont(UI_10_FONT_ID, ui10FontFamily);
   renderer.insertFont(UI_12_FONT_ID, ui12FontFamily);
   renderer.insertFont(SMALL_FONT_ID, smallFontFamily);
+
+  // Discover and load SD card fonts
+  sdFontSystem.begin(renderer);
+
   LOG_DBG("MAIN", "Fonts setup");
 }